This invention relates to assembling two workpieces by the use of ultrasonic vibrations and, more specifically, concerns the assembling of workpieces by the so-called ultrasonic staking or riveting process wherein ultrasonic energy supplied by a horn is used to deform a thermoplastic stud upstanding from a first workpiece to capture a second workpiece having an aperture through which such study extends. This process is described, for example, in "Ultrasonic Plastics Assembly" published by Branson Sonic Power Company, Danbury, CT 06810 (1979), pages 49-51.
Ultrasonic staking has been revealed in U.S. Pat. No. 3,367,809 dated Feb 6, 1968 issued to R. S. Soloff entitled "Sonics" and in U.S. Pat. No. 3,499,808 dated Mar 10, 1970 issued to E. G. Obeda entitled "Method and apparatus for Securing Parts Together by Sonic Energy".
The process has found wide acceptance in almost all instances where thermoplastic parts or a thermoplastic part and a non-thermoplastic part are to be assembled or fastened to one another. There are numerous applications of this type in the manufacture of electrical appliances, automobile dashboards and, more recently, in assembling watch movements and digital computer components.
In many instances, there exists the requirement to more accurately control, during the deformation of the stud, the flow of the softened thermoplastic material in order to provide a mechanically precise assembly and also an assembly characterized by a neat and pleasing appearance. The uncontrolled flow of thermoplastic material is particularly undesirable when crystalline and filled plastic materials are used. Typical fillers include, glass, talc, calcium carbonate, carbon, etc.. With the use of such fillers there is a tendency for the generation of particulate matter and loose particles which crumble and fall away from the assembly. With amorphous plastics there is a tendency for the softened thermoplastic material to flow too freely.